Io is the innermost of the four moons. It is a small world, 3620 km in diameter and orbits Jupiter at a distance of 422,000 km. in 1.7 days. Io's density is 3.55 g/cm3 which indicates that it is mostly composed of rocky material.

     Io's surface shows no impact craters. The surface appears to be a sulfur desert with sulfur ash raining down from the sky. This sulfur ash rain results from Io's active volcanoes. Io was the first world other than Earth where active volcanism was known to exist.

     Io's volcanoes are not caused by the same thing that causes volcanoes here on Earth. It is tidal heating that creates volcanoes on Io. Europa and Ganymede affect Io's orbit, causing it to be slightly elliptical. This makes Io's distance from Jupiter vary and this causes Io to be "pushed" and "pulled" from several directions! It is this flexing that causes heat to be generated in Io's interior which in turn melts rocks and causes this melted material to rise to the surface. This rising melted rock melts the sulfur above and causing sulfur and sulfur dioxide gas to vent through the surface, as a volcano. We see the liquid and solid sulfur as well as the white frost of sulfur dioxide gas that coat Io's surface. These four images show changes over time to the volcano Ra Patera. Both top images were taken by the Voyager spacecraft in 1979. The lower left was taken by the Galileo spacecraft in 1996, and the lower right was taken by Voyager 2 in 1979. Io is indeed a world torn inside out!